The development of high-power, high-energy, long-life, and low-cost rechargeable batteries is critical for the next-generation electric and hybrid electric vehicles. Among various battery technologies, lithium-ion (or Li-ion) batteries are promising energy storage devices as a result of the high energy densities, low self-discharges, and long cycle lives of known Li-ion batteries.
Three basic functional elements support the electrochemical reactions in a lithium ion battery. These elements are anode, cathode, and electrolyte. Both the anode and cathode are materials into which and from which lithium ions can migrate. The process of lithium moving into the anode or cathode is referred to as insertion (or intercalation), and the reverse process, in which lithium moves out of the anode or cathode is referred to as extraction (or deintercalation). When a cell is discharging, the lithium ions are extracted from the anode and inserted into the cathode. When the cell is charging, the reverse process occurs: lithium ions are extracted from the cathode and inserted into the anode.
The negative electrode during discharge (the anode) of a conventional and commercially available Li-ion cell has typically been made from graphite. The positive electrode during discharge (the cathode) is conventionally made of a metal oxide such as LiCoO2. The electrolyte is typically a lithium salt in an organic solvent, for example, LiPF6 dissolved in ethylene carbonate/diethyl carbonate.
The Li ion insertion and extraction processes at the electrodes in a Li-ion battery are typically described by the following reaction:xLi−+xe−+M LixM   (1)where M represents the electrode material. Despite the commercial success of Li-ion batteries to date, the performance of microstructured electrodes, such as graphite, is limited by several factors, including but not limited to: slow charge/discharge rates resulting from long lithium diffusion lengths in the electrode materials; structural instability induced by crystal lattice strain arising from lithium insertion/extraction processes; and irreversible Li ion capacity loss owing, in part, to structure instability.
Advances have been made in the fields of nanoscience and nanotechnology which allow for the nanoscale structuring of Li-ion electrode materials. Compared with conventional microstructured electrode materials, the small structural elements of nanostructured electrodes can result in much shorter solid-state lithium diffusion lengths (shorter Li-insertion distances), leading to faster charge and discharge rates and, therefore, higher power densities. Also, nanostructured materials can sustain a higher degree of strain during the lithium insertion/extraction processes, permitting a larger number of charge and discharge cycles with improved capacity-retention capability. The large electrode/electrolyte contact area of certain nanostructured electrodes reduces the interfacial Li insertion/extraction current density, enabling further improvements in the rate capability of a battery. Newly observed Li ion storage mechanisms, which are relatively unimportant in bulk or microstructured electrodes, become substantive on the nanoscale. For example, in addition to the normal Li ion storage mechanism of the bulk material, a large surface capacitive effect associated with nanomaterials, such as nanostructured oxides, can substantially increase Li ion storage capability.
Known nanostructured electrodes feature inherently disordered or randomly-packed materials, such as randomly grown or applied nanoparticles, nanowires, or nanotubes. Although certain advantages exist with nanostructured electrode materials, the disordered or random packing of known nanostructures generally leads to limiting operational characteristics, including but not limited to: convoluted and relatively long electron/ion conducting pathways; losses of surface area owing to agglomerations; and low-packing densities in the case of non-oriented nanowires, or nanotubes. These structure-related issues limit the power and energy densities achievable with a battery or other device using randomly packed nanoscale electrode materials.